Why can I not map the Esc. key to "Show/Hide Viewer on Second Display" in FCP?

While editing in FCP, I like to show the Viewer on my second monitor. To do this, I have to click the little Show/Hide... monitor button in the top right of FCP, or go to: Window > Show in Secondary Display > Viewer. Then to pop out of that view, I have to do the same thing. I'd like to map that feature to the Esc. key, so that I can toggle back and forth between that "view".

So, I went to: Final Cut Pro > Commands > Customize... using the graphic keyboard that pops up, I find the Command labeled: Show/Hide Viewer on Second Display. I press the Esc key to see it, and I see the little "Esc" icon show up in the "Key" column, No Modifier. I click Save... but... the key command does not work. When I press the Esc key, I just hear that annoying little plop sound that you hear when you press an invalid key on a Mac.

I tried restarting FCP. Still doesn't work.

I tried to reopen Final Cut Pro > Commands > Customize... and the Esc. key command did "Save" properly for: "Show/Hide Viewer on Second Display". But... it still just doesn't work. Is there some kind of rules with FCP that some keys are just "off limits" for keyboard commands, because they would conflict with the OS? If so... why would the FCP Custom Commands feature let me Save it... but not use it?